Carboxymethylated microfibrillated cellulose fibers and composition thereof

ABSTRACT

Provided is a carboxymethylated microfibrillated cellulose fiber having a Canada standard freeness of less than 200 mL and an average fiber diameter of not less than 500 nm. Said fiber provides a composition having excellent water retention ability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage filing, under 35 U.S.C. §371(c), of International Application No. PCT/JP2019/013630, filed onMar. 28, 2019, which, in turn, claims priority to Japanese PatentApplication No. 2018-070263, filed on Mar. 30, 2018.

TECHNICAL FIELD

The present invention relates to a carboxymethylated microfibrillatedcellulose fiber and a composition comprising said fiber.

BACKGROUND ART

During a papermaking process, a composition prepared by dispersing apulp and a pigment in water is used. The water retention ability of sucha composition is important from the viewpoints of increased efficiencyof production process and improvement of product quality. For example,when a base paper is made using a pulp slurry as a raw material, thewater retention ability of the pulp slurry has a great impact on thewater drainage of the slurry on a wire screen and the dispersibility ofthe pulp, and as a consequence on the paper strength, air resistance andbulkiness of a produced paper. Further, the degree of penetration of abinder into a base paper varies depending on the water retention abilityof a pigment coating liquid, and thus, the water retention ability of apigment coating liquid has a great impact on the strength andadhesiveness of a pigment coated layer and a base paper. In recentyears, many studies have been actively conducted on cellulose nanofibersmade using cellulose as a raw material. For example, PTL 1 discloses atechnique related to a composition comprising a cellulose nanofiber.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. JP2017-110085

SUMMARY OF INVENTION Technical Problem

The present inventors conceived an idea that if the water retentionability of a composition can be enhanced by using a microfibrillatedcellulose fiber with a lower degree of defibration than cellulosenanofibers, papers with increased strength can be produced with lowcost. However, no study of such an idea has been conducted yet. In lightof these circumstances, an object of the present invention is to providea microfibrillated cellulose fiber that provides a composition having anexcellent water retention ability and exerts an effect to enhance paperstrength when added to a paper.

Solution to Problem

The present inventors found that carboxymethylated microfibrillatedcellulose fibers with a particular level of freeness can achieve theaforementioned object. Therefore, the aforementioned object is achievedby the following invention.

(1) A carboxymethylated microfibrillated cellulose fiber having a Canadastandard freeness of less than 200 mL and an average fiber diameter ofnot less than 500 nm.

(2) The carboxymethylated microfibrillated cellulose fiber as set forthin (1), having an electrical conductivity of not more than 500 mS/m, asmeasured at pH 8 in the form of a 1% by weight solids concentrationwater dispersion.

(3) The carboxymethylated microfibrillated cellulose fiber as set forthin (1) or (2), having a degree of substitution of from 0.01 to 0.50.

(4) The carboxymethylated microfibrillated cellulose fiber as set forthin any of (1) to (3), having a cellulose type-I crystallinity of notless than 50%.

(5) A composition comprising the carboxymethylated microfibrillatedcellulose fiber as set forth in any of (1) to (4) and water.

(6) The composition as set forth in (5), further comprising a rawmaterial pulp.

(7) The composition as set forth in (5) or (6), further comprising abinder.

(8) The composition as set forth in any of (5) to (7), furthercomprising a white pigment.

(9) A dry solid formed by drying the composition as set forth in any of(5) to (8).

(10) A method of preparing the carboxymethylated microfibrillatedcellulose fiber as set forth in any of (1) to (4), the method comprisingthe steps of:

(A) carboxymethylating a pulp,

(B) wet-grinding the pulp.

Advantageous Effects of Invention

According to the present invention, there can be provided amicrofibrillated cellulose fiber that provides a composition having anexcellent water retention ability and exerts an effect to enhance paperstrength when added to a paper.

DESCRIPTION OF EMBODIMENTS

The present invention provides a carboxymethylated microfibrillatedcellulose fiber having a Canada standard freeness of less than 200 mLand an average fiber diameter of not less than 500 nm. In thisinvention, ranges “from X to Y” include both endpoints X and Y.

1. Carboxymethylated Microfibrillated Cellulose Fiber

(1) Carboxymethylated Microfibrillated Cellulose Fiber

Microfibrillated cellulose (hereinafter also referred to as “MFC”)fibers refer to fibers having an average fiber diameter (also referredto as “average fiber width”) of not less than 500 nm, which are obtainedby fibrillating a cellulose-based raw material such as pulp.Carboxymethylated microfibrillated cellulose (hereinafter also referredto as “CM-modified MFC”) fibers refer to a MFC obtained by fibrillatinga CM-modified cellulose-based raw material. As described later, thefibrillation is preferably carried out by mechanical treatment. However,considering that a CM-modified pulp is easy to ravel, it is consideredthat the CM-modified pulp is more or less fibrillated since fibers arefinely broken or loosened through a series of CM process steps (esp.,dehydration and washing after carboxymethylation). Therefore, in thisinvention, the CM-modified MFC also includes a CM-modified pulp notsubjected to mechanical treatment. In this invention, the average fiberdiameter refers to a length-weighted average fiber diameter, which canbe determined by an image analysis-based fiber analyzer, such as a fibertester produced by ABB Japan K.K. or a fractionator produced by ValmetK.K. For example, the MFC is obtained by relatively gently defibratingor beating a cellulose-based raw material using a beater, disperser orthe like. Therefore, the MFC has a larger fiber diameter than cellulosenanofibers obtained by intensely defibrating a cellulose-based rawmaterial by a high pressure homogenizer or the like, and has a structurein which the fiber surface is efficiently fluffed (externallyfibrillated) while the fiber itself is left not microfiberized(internally fibrillated).

As mentioned above, the CM-modified MFC of the present invention may bea CM-modified pulp obtained by chemically modifying (carboxymethylating)a pulp, but is preferably a mechanically-treated, CM-modifiedmicrofibrillated cellulose fiber (hereinafter also referred to as“mechanically-treated, CM-modified MFC”) obtained by subjecting aCM-modified pulp to further mechanical treatment such as defibration. Inother words, the CM-modified MFC of this invention has electricallycharged carboxymethyl groups on the fiber surface regardless of beingsubjected to mechanical treatment, and thus, as compared to unmodifiedpulp, shows an increased water retention value and varying affinity fordifferent chemicals. However, the mechanically-treated, CM-modified MFCobtained by mechanically treating a CM-modified pulp has an increasedspecific surface area and thus enables the effects of this invention tobe produced at high levels. Since the mechanically-treated, CM-modifiedMFC is obtained by relatively gently defibrating or beating aCM-modified cellulose-based raw material such as CM-modified pulp,strong hydrogen bonding present between fibers is weakened by chemicalmodification. Thus, as compared to a MFC obtained simply by mechanicaldefibration or beating, the mechanically-treated, CM-modified MFC ischaracterized in that the fibers are easier to ravel, less damaged, andinternally and externally fibrillated in a moderate manner. Further, awater dispersion obtained by dispersing the CM-modified MFC of thisinvention in water has high hydrophilicity, high water retentionability, and high viscosity.

As mentioned above, the MFC differs in degree of fibrillation from acellulose-based raw material. It is generally not easy to quantify adegree of fibrillation, but the present inventors found that the degreeof fibrillation of a MFC can be quantified based on its Canada standardfreeness, water retention value, and transparency.

The CM-modified MFC of the present invention has a carboxymethyl group,which is an anionic group introduced thereto, and thus the physicalproperties of the CM-modified MFC, including affinity for water, varydepending on the type of carboxymethyl groups, i.e., whether thecarboxymethyl groups are of H-type or of salt type. The properties ofthe CM-modified MFC can vary since the type of carboxymethyl groups isadjusted as appropriate depending on the intended use. Unless otherwisestated, the fiber properties of the CM-modified MFC of this inventionare evaluated based on the measurements obtained for a CM-modified MFCwhich provides an alkaline water dispersion, or more specifically awater dispersion with 1% by weight concentration at pH 8. Since theCM-modified MFC, unlike common pulp, has anionic substituents introducedthereto, said MFC can advantageously be used as an additive such asdispersant or coagulant, taking advantage of its anioniccharacteristics.

<Canada Standard Freeness>

The Canada standard freeness of the CM-modified MFC of the presentinvention, as measured so as to ensure that the aforementionedrequirements are met, is less than 200 mL, preferably not more than 150mL. The lower limit of the freeness is not limited but is preferablyhigher than 0 mL. The Canada standard freeness of the CM-modified MFC ofthis invention can be adjusted by adjusting the degrees of processinginto short fibers, nanosizing and fibrillation during treatment of aCM-modified pulp used as a raw material. As a result of intensivestudies, the present inventors found that by fibrillating a CM-modifiedpulp to reduce Canada standard freeness, a novel material different fromcommon chemical pulp or cellulose nanofiber (also referred to as “CNF”)can be obtained. In other words, since the CM-modified MFC of thisinvention has a Canada standard freeness of less than 200 mL, saidCM-modified MFC is presumed to have a form of fully fibrillated fibers.Further, since a CNF obtained by intensely defibrating a fiber tomicronize it to the single-nanometer level has a Canada standardfreeness of 0 mL, the CM-modified MFC of this invention differs from aCNF. The CM-modified MFC having a Canada standard freeness of less than200 mL has an excellent water retention ability and can advantageouslybe used as a water retaining material. Further, when the CM-modified MFCof this invention is used as a papermaking additive, fibrils formed onthe surface of the CM-modified MFC may contribute to an increase in thepoints of bonding between cellulose fibers, leading to formation of astrong network between CM-modified MFCs or between a CM-modified MFC anda pulp, whereby a paper with enhanced barrier properties and paperstrength can be produced.

<Water Retention Value>

The water retention value of the CM-modified MFC of the presentinvention is preferably not less than 300%. When the water retentionvalue is less than 300%, the effect of this invention, which isenhancing the water retention ability of a composition comprising theCM-modified MFC of this invention, may not be fully obtained. Waterretention value is determined according to JIS P-8228:2018.

<Transparency in the Form of Water Dispersion>

The CM-modified MFC of the present invention is characterized by havinglow transparency when made into the form of a water dispersion preparedusing water as a dispersion medium. In this invention, the transparencyrefers to the transmittance of light at a wavelength of 660 nm through a1% (w/v) solids concentration water dispersion of a material of interest(e.g., CM-modified MFC). The specific method of determining transparencyis as described below.

A dispersion of the CM-modified MFC (solids concentration: 1% (w/v),dispersing medium: water) is prepared and determined for transmittanceof light at 660 nm using the UV-VIS spectrophotometer UV-1800 (producedby Shimadzu Corporation) equipped with rectangular cells with an opticalpath length of 10 mm.

In the present invention, the transparency of the CM-modified MFC ispreferably not more than 40%, more preferably not more than 30%, stillmore preferably not more than 20%, yet more preferably not more than10%. In general, the transparency of a cellulose-based materialincreases when the material is nanosized while it retains itscrystallinity. In contrast, the CM-modified MFC of this invention has alow level of transparency because said MFC is not so much highlynanosized and retains its fiber structure. When the CM-modified MFChaving a transparency of not more than 40% is internally incorporatedinto a paper, the CM-modified MFC retains its fiber structure in thepaper, thereby reducing the occurrence of a decline in paper thicknessor paper density and enabling enhancement of paper strength withoutdeterioration of rigidity.

<Electrical Conductivity>

The electrical conductivity of the CM-modified MFC of the presentinvention is preferably not more than 500 mS/m, more preferably not morethan 300 mS/m, still more preferably not more than 200 mS/m, yet morepreferably not more than 100 mS/m, most preferably 70 mS/m, as measuredunder the condition of pH 8 in the form of a 1.0% by weight waterdispersion. The lower limit of the electrical conductivity is preferablynot less than 5 mS/m, more preferably not less than 10 mS/m. Theelectrical conductivity of the CM-modified MFC is higher than that of aCM-modified cellulose-based material used as a raw material. Anelectrical conductivity exceeding the upper limit means that theconcentration of metal and inorganic salts dissolved in a waterdispersion of a CM-modified cellulose-based material is above aspecified value. When the concentration of metal and inorganic salts insaid material is low, electrostatic repulsion can easily occur betweenfibers, promoting efficient fibrillation.

Hereunder, a method of preparing a CM-modified MFC will be described.

1) Cellulose-Based Raw Material

Examples of cellulose-based raw materials include, but are notparticularly limited to, cellulose-based raw materials derived fromplants, animals (e.g., sea squirt), algae, microorganisms (e.g.,Acetobacter), and microorganism products. Examples of cellulose-basedraw materials derived from plants include wood, bamboo, hemp, jute,kenaf, farm waste products, cloth, and pulps (e.g., softwood (nadelholz)unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP),hardwood (laubholz) unbleached kraft pulp (LUKP), hardwood bleachedkraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwoodbleached sulfite pulp (NBSP), thermomechanical pulp (TMP), softwooddissolving pulp, hardwood dissolving pulp, recycled pulp, waste paper).Also, a cellulose powder obtained by grinding such a cellulose-based rawmaterial as mentioned above may be used. The cellulose raw material usedin the present invention can be any or a combination of theaforementioned materials, but is preferably a cellulose fiber derivedfrom a plant or microorganism, more preferably a cellulose fiber derivedfrom a plant, still more preferably a wood-based pulp, most preferably ahardwood pulp.

The average fiber diameter of a cellulose fiber is not particularlylimited. Commonly used softwood kraft pulps have an average fiberdiameter of about from 30 to 60 μm, and hardwood kraft pulps have anaverage fiber diameter of about from 10 to 30 μm. Other pulps after acommon purification procedure have an average fiber diameter of about 50μm. For example, in the case of using a raw material obtained throughpurification of a several centimeter-sized material such as chip, it ispreferable to subject the raw material to mechanical treatment by adisintegrator such as refiner or beater to adjust average fiber diameterto not more than about 50 μm, more preferably not more than about 30 μm.

2) Carboxymethylation

Carboxymethylation refers to introducing carboxymethyl groups into acellulose-based raw material via ether bonds. The carboxymethyl groupsmay be introduced in the form of a salt (—CH₂—COOM, where M is a metalatom). Carboxymethylation is also referred to as etherification. Thefollowing provides a detailed description of etherification.

<Cellulose Type-I Crystallinity>

With regard to the cellulose crystallinity of the CM-modified MFC of thepresent invention, type-I crystals are preferably present at aconcentration of not less than 50%, more preferably not less than 60%.By adjusting crystallinity within the aforementioned range, theCM-modified MFC can exhibit effects, including imparting water retentionproperties, when added to a paper. Further, when type-I crystals arepresent at a concentration of not less than 50% in a CM-modified pulp asa raw material, the pulp can be efficiently fibrillated by beating ordefibrating treatment while it retains its fiber structure, whereby theCM-modified MFC of this invention can be prepared efficiently. Cellulosecrystallinity can be controlled by the concentration of a mercerizingagent, the temperature of mercerization treatment, and the degree ofcarboxymethylation. Since a high concentration of alkali is used formercerization and carboxymethylation, cellulose type-I crystals arelikely to convert to type-II. However, by controlling the degree ofmodification through, for example, adjusting the amount of an alkali(mercerizing agent) used, desired crystallinity can be maintained. Theupper limit of cellulose type-I crystallinity is not particularlylimited. In practice, said upper limit is presumed to be about 90%.

The method of determining the cellulose type-I crystallinity of aCM-modified MFC is as described below.

A sample is placed into a glass cell and subjected to measurement usingan X-ray diffractometer (LabX XRD-6000, produced by ShimadzuCorporation). The calculation of crystallinity is performed by a methodsuch as Segal—the crystallinity is calculated according to the followingequation based on the diffraction strength of plane (002) at 2θ=22.6°and the diffraction strength of amorphous region at 2θ=18.5°, with thediffraction strength at 2θ=10° to 30° in an X-ray diffraction diagrambeing used as a baseline.Xc=(I _(002c) −I _(a))/I _(002c)×100

Xc: Cellulose type-I crystallinity (%)

I_(002c): Diffraction strength of plane (002) at 2θ=22.6°

I_(a): Diffraction strength of amorphous region at 2θ=18.5°.

CM-modified cellulose can generally be prepared by treating(mercerizing) a cellulose material with alkali and then reacting theresulting mercerized cellulose (also referred to as “alkali cellulose”)with a carboxymethylating agent (also referred to as “etherifyingagent”). In the thus-obtained CM-modified cellulose, any of the hydroxylgroups located at C2, C4 and C6 position in pyranose rings iscarboxymethylated. In general, carboxymethylcellulose (CMC), which isformed by dry-grinding a CM-modified cellulose, is characterized byhaving water swellability, high safety and the like, and is used as anadditive for cosmetics and food products. Therefore, similarly to CMC,the CM-modified MFC of the present invention, which is prepared using aCM-modified cellulose as a raw material, can also be advantageously usedas an additive for food products and cosmetics.

The degree of carboxymethyl substitution per anhydrous glucose unit in aCM-modified cellulose or MFC obtained by carboxymethylation ispreferably not less than 0.01, more preferably not less than 0.05, stillmore preferably not less than 0.10. The upper limit of this degree ispreferably not more than 0.60, more preferably not more than 0.50, stillmore preferably not more than 0.4. Therefore, the degree ofcarboxymethyl substitution is in the range of preferably from 0.01 to0.50, more preferably from 0.05 to 0.40, still more preferably from 0.10to 0.30. In general, a CM-modified cellulose has higher affinity forwater and higher swellability in water as the degree of carboxymethylsubstitution becomes higher and the cellulose type-I crystallinitybecomes lower. However, the present inventors found that when aCM-modified pulp obtained by performing a carboxymethylation reactionwithout deteriorating crystallinity is used as a raw material and theCM-modified pulp is beaten or defibrated in a high water content state,there can be obtained a CM-modified MFC which is fibrillated whileretaining its fiber structure.

The method of carboxymethylation is not particularly limited, andexamples thereof include such a method as mentioned above, in which acellulose raw material used as a starting material is subjected tomercerization followed by etherification. For the carboxymethylationreaction, a solvent is generally used. Examples of a solvent includewater, alcohols (e.g., lower alcohol) and mixed solvents thereof.Examples of a lower alcohol include methanol, ethanol, N-propyl alcohol,isopropyl alcohol, N-butanol, isobutanol, and tertiary butanol. As forthe mixing proportion of a lower alcohol in a mixed solvent, the lowerlimit is generally not less than 60% by weight, and the upper limit isnot more than 95% by weight—thus, said mixing proportion is preferablyin the range of from 60 to 95% by weight. The amount of a solvent isgenerally 3 times by weight that of the cellulose raw material. Theupper limit of this amount is not particularly limited, but ispreferably 20 times by weight. Therefore, the amount of a solvent ispreferably in the range of from 3 to 20 times by weight.

Mercerization is generally performed by mixing a starting material witha mercerizing agent. Examples of a mercerizing agent include alkalimetal hydroxides such as sodium hydroxide and potassium hydroxide. Theamount of a mercerizing agent used is preferably not less than 0.5 timesmoles, more preferably not less than 1.0 times mole, still morepreferably not less than 1.5 times moles, per anhydrous glucose residuesin a starting material. The upper limit of this amount is generally notmore than 20 times moles, preferably not more than 10 times moles, morepreferably not more than 5 times moles. Therefore, the amount of amercerizing agent used is in the range of preferably from 0.5 to 20times moles, more preferably from 1.0 to 10 times moles, still morepreferably from 1.5 to 5 times mole.

The reaction temperature for mercerization is generally not less than 0°C., preferably not less than 10° C., and the upper limit of thisreaction temperature is generally not more than 70° C., preferably notmore than 60° C. Therefore, this reaction temperature is generally inthe range of from 0 to 70° C., preferably from 10 to 60° C. The reactiontime for mercerization is generally not less than 15 minutes, preferablynot less than 30 minutes. The upper limit of this reaction time isgenerally not more than 8 hours, preferably not more than 7 hours.Therefore, this reaction time is generally in the range of from 15minutes to 8 hours, preferably from 30 minutes to 7 hours.

The etherification reaction is generally performed by adding acarboxymethylating agent to the reaction system after mercerization.Examples of a carboxymethylating agent include sodium monochloroacetate.The amount of a carboxymethylating agent added is generally preferablynot less than 0.05 times moles, more preferably not less than 0.5 timesmoles, still more preferably not less than 0.8 times moles, per glucoseresidues in a cellulose raw material. The upper limit of this amount isgenerally not more than 10.0 times moles, preferably not more than 5times moles, more preferably not more than 3 times moles. Therefore,this amount is in the range of preferably from 0.05 to 10.0 times moles,more preferably from 0.5 to 5 times moles, still more preferably from0.8 to 3 times moles. The reaction temperature for etherification isgenerally not less than 30° C., preferably not less than 40° C., and theupper limit of this reaction temperature is generally not more than 90°C., preferably not more than 80° C. Therefore, this reaction temperatureis generally in the range of from 30 to 90° C., preferably from 40 to80° C. The reaction time for etherification is generally not less than30 minutes, preferably not less than 1 hour, and the upper limit of thisreaction time is generally not more than 10 hours, preferably not morethan 4 hours. Therefore, this reaction time is generally in the range offrom 30 minutes to 10 hours, preferably from 1 to 4 hours. During thecarboxymethylation reaction, a reaction solution may be stirreddepending on the need.

The degree of carboxymethyl substitution per glucose unit in aCM-modified cellulose is determined according to, for example, thefollowing method: 1) about 2.0 g (absolute dry) of a CM-modifiedcellulose is precisely weighted out and placed into a 300 mL stopperedconical flask; 2) 100 mL of a mixed solution of 1000 mL of methanol and100 mL of premium grade concentrated nitric acid is added, and shakingis continued for 3 hours to convert a carboxymethylcellulose salt(CM-modified cellulose) to a H-type CM-modified cellulose; 3) 1.5-2.0 gof the H-type CM-modified cellulose (absolute dry) is precisely weightedout and placed into a 300 mL stoppered conical flask; 4) the H-typeCM-modified cellulose is wetted with 15 mL of 80% methanol, 100 mL of0.1 N NaOH is added, and shaking is continued at room temperature for 3hours; 5) excess NaOH is back titrated with 0.1 N H₂SO₄ usingphenolphthalein as an indicator; 6) the degree of carboxymethylsubstitution (DS) is calculated according to the following equation.A=[(100×F′−(0.1 N H₂SO₄)(mL)×F)×0.1]/(absolute dry mass (g) of H-typeCM-modified cellulose)

DS=0.162×A/(1−0.058×A)

A: Amount (mL) of 1 N NaOH required for neutralization of 1 g of H-typeCM-modified cellulose

F: Factor for 0.1 N H₂SO₄

F′: Factor for 0.1 N NaOH

3) Mechanical Treatment

At this step, a CM-modified pulp is mechanically defibrated, beaten ordisintegrated to an average fiber diameter of not less than 500 nm.Mechanical defibration, beating or disintegration is referred to as“mechanical treatment”, and defibrating or beating a CM-modified pulpdispersed in water is referred to as “wet-grinding”. Mechanicaltreatment may be performed once, or may be performed two or more timesby repeating the same procedure or combining different procedures. Inthe case of performing mechanical treatment two or more times, differentprocedures may be performed at any given timing, and the apparatus to beused may be the same or different. This step can be performed, forexample, by any of the following procedures.

the water dispersion of a CM-modified pulp is concentrated to highconcentration (not less than 20% by weight) by dehydration or the like,and then subjected to beating;

the water dispersion of a CM-modified pulp is diluted to reduceconcentration (less than 20% by weight, preferably not more than 10% byweight), and then subjected to mechanical treatment such as beating ordefibration;

the CM-modified pulp is subjected to drying, followed by mechanicaldefibration or beating;

the CM-modified pulp is subjected to preliminary dry-grinding into shortfibers, followed by mechanical defibration or beating.

Since the present invention aims at promoting fibrillation of a fiberwhile preventing it from being processed into short fibers, it ispreferable to perform mechanical treatment once, and more preferable totreat a low-concentrated water dispersion of a CM-modified pulp using arefiner or a high-speed disintegrator.

The apparatus used for mechanical treatment is not particularly limited,and examples thereof include different types of apparatus, such ashigh-speed rotating type, colloid mill type, high pressure type, rollmill type, and ultrasonic type. Specific examples thereof that can beused include some types of apparatus which perform mechanical treatmentby causing a metal or blade moving around the axis of rotation on pulpfibers, and other types of apparatus which perform mechanical treatmentby means of the friction between pulp fibers, as exemplified byhigh-pressure or ultrahigh-pressure homogenizer, refiner, beater, PFImill, kneader, disperser, and high-speed disintegrator.

In the case of defibrating or beating a CM-modified pulp dispersed inwater, the lower limit of the solids concentration of the CM-modifiedpulp in the water dispersion is generally preferably not less than 0.1%by weight, more preferably not less than 0.2% by weight, still morepreferably not less than 0.3% by weight. At such a solids concentration,the relative amount of a dispersion medium to the amount of theCM-modified pulp becomes appropriate, leading to greater efficiency. Theupper limit of this concentration is generally preferably not more than50% by weight.

At this step, a CM-modified MFC is obtained. The average fiber diameterof a CM-modified MFC is not less than 500 nm, preferably not less than 1μm, more preferably not less than 10 μm, in terms of length-weightedaverage fiber diameter. The upper limit of the average fiber diameter ispreferably not more than 60 μm, more preferably not more than 40 μm. Theaverage fiber length of a CM-modified MFC is preferably not less than300 μm, more preferably not less than 400 μm, in terms oflength-weighted average fiber length. The upper limit of the averagefiber length is preferably not more than 3000 μm, more preferably notmore than 1500 μm, still more preferably not more than 1100 μm, mostpreferably not more than 900 μm. According to the present invention,since a raw material pulp is carboxymethylated beforehand and subjectedto mechanical treatment by a procedure that minimizes cutting of fibers,the pulp fibers can be fibrillated without being cut to an extremelyshort length. Further, since the affinity of cellulose fibers for wateris increased by carboxymethylation, the CM-modified MFC can exhibit lowfreeness in spite of having a long fiber length.

Length-weighted average fiber diameter and length-weighted average fiberlength can be determined using an image analysis-based fiber analyzer,such as a fiber tester produced by ABB Japan K.K. or a fractionatorproduced by Valmet K.K. The average aspect ratio of a CM-modified MFC ispreferably not less than 10, more preferably not less than 30. The upperlimit of the average aspect ratio is not particularly limited, and ispreferably not more than 1000, more preferably not more than 100, stillmore preferably not more than 80. The average aspect ratio can becalculated according to the following equation.Average aspect ratio=average fiber length/average fiber diameter

It is preferable that the degree of substitution per glucose unit in theCM-modified MFC obtained at this step should be the same as that of aCM-modified pulp used as a raw material.

2. Composition

The composition of the present invention comprises a CM-modified MFC andwater. The composition of this invention, which comprises a CM-modifiedMFC and water as mentioned above, can be widely used for anyapplications that require water retention. The composition of thisinvention can be used to serve as, for example, a thickener, a gellant,a shape retainer, an emulsion stabilizer, or a dispersion stabilizer. Tobe specific, the composition of this invention can be used inpapermaking raw materials (additive, raw material pulp), food products,cosmetics, pharmaceuticals, agricultural chemicals, toiletries, sprays,paints, and the like. However, it is preferred that the composition ofthis invention should be used, in a paper production process, as a paperraw material (pulp slurry) for use at a papermaking step or as a pigmentcoating liquid or clear coating liquid for use at a coating step. Thus,these applications are described below for instance.

(1) Pulp slurry

A pulp slurry comprises not only a CM-modified MFC and water, but also araw material pulp. The raw material pulp refers to a pulp that serves asa main component of a paper. The pulp raw material for a base paper usedin the present invention is not particularly limited, and examplesthereof that can be used include: mechanical pulps such as ground pulp(GP), thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP);waste paper pulps such as deinked pulp (DIP) and undeinked pulp; andchemical pulps such as nadelholz (softwood) kraft pulp (NKP) andlaubholz (hardwood) kraft pulp (LKP). As waste paper pulps, use can bemade of those pulps derived from sorted waste papers such ashigh-quality paper, medium-quality paper, low-quality paper, newspaperwaste paper, leaflet waste paper, magazine waste paper, corrugatedpaper, and printed waste paper, or those pulps derived from unsortedwaste papers comprising a mixture of different waste papers.

The content of a CM-modified MFC in a pulp slurry is preferably 1×10⁻⁴to 20% by weight, more preferably 1×10⁻³ to 5% by weight, based on theamount of a raw material pulp. If this content exceeds its upper limit,the water retention ability of the pulp slurry will become too high,possibly causing poor water drainage at a papermaking step. If thiscontent falls below its lower limit, enhancement of water retentionability or enhancement of the paper strength of a produced paper may notbe achieved due to too small an amount of a CM-modified MFC added.

The pulp slurry may contain a known filler. Examples of fillers include:inorganic fillers such as heavy calcium carbonate, light calciumcarbonate, clay, silica, light calcium carbonate-silica composite,kaolin, fired kaolin, delaminated kaolin, magnesium carbonate, bariumcarbonate, barium sulfate, aluminum hydroxide, calcium hydroxide,magnesium hydroxide, zinc hydroxide, zinc oxide, titanium oxide, andamorphous silica produced by neutralizing sodium silicate with a mineralacid; and organic fillers such as urea-formalin resin, melamine resin,polystyrene resin and phenol resin. Such fillers may be used alone or incombination. Among them, preferred is heavy calcium carbonate or lightcalcium carbonate, which are representative fillers used to make neutraland alkaline papers and can give papers high opacity. The content of afiller is in the range of preferably from 5 to 20% by weight based onthe amount of a raw material pulp. In the present invention, it is morepreferred that the content of a filler should be not less than 10% byweight, since the decline in paper strength can be reduced even whenpaper ash content is high.

The CM-modified MFC of the present invention can function as a paperstrengthening agent, a water retainer, or a yield improver in a pulpslurry. In addition to the MFC of this invention, various wet endadditives, including bulking agent, dry paper strengthening agent, wetpaper strengthening agent, freeness improver, dye, or cationic, nonionicor anionic sizing agent, may be added to a pulp slurry depending on theneed.

The pulp slurry of the present invention is prepared by any givenmethod, but it is preferable to add the CM-modified MFC at the step ofsubjecting a raw material pulp to refining or mixing treatment. When theCM-modified MFC is added at a mixing step, a mixture prepared beforehandby mixing the CM-modified MFC with a filler and other auxiliary agentssuch as yield improver may be added to a raw material pulp slurry.

The solids concentration of a pulp slurry is adjusted as appropriatedepending on papermaking conditions and the like, but is preferably inthe range of from 0.1 to 1.0% by weight. Such a pulp slurry is made intoa paper by a known papermaking method. Papermaking can be carried outusing, for example, but not limited to, a fourdrinier paper machine, agap former-type paper machine, a hybrid former-type paper machine, anon-top former-type paper machine, or a cylinder paper machine.

(2) Clear Coating Liquid

The clear coating liquid is a coating liquid composed mainly of awater-soluble polymer commonly used as a surface treating agent,including starch (e.g., oxidized starch, modified starch, dextrin),carboxymethylcellulose, polyacrylamide, or polyvinyl alcohol. Inaddition to the water-soluble polymer, various additives such as waterresisting agent, external sizing agent, surface strengthening agent, dyeor pigment, fluorescent colorant, and water retainer may be contained ina clear coating liquid. The water-soluble polymer can also serve as abinder.

The content of a CM-modified MFC in a clear coating liquid is notparticularly limited. Total solids content may consist of a CM-modifiedMFC, but from viewpoints of coating suitability and the like, it ispreferred to use a CM-modified MFC in admixture with the aforementionedwater-soluble polymer. The mixing ratio of water-soluble polymer andCM-modified MFC is in the range of preferably from 1:10000 to 10000:1,more preferably about from about 1:1 to 500:1.

By coating one or both sides of a base paper with a clear coating liquidby a known method, a clear coating layer can be formed. In the presentinvention, the term “clear coating” refers to coating or impregnating abase paper with a clear coating liquid using a coater such as sizepress, gate roll coater, premetered size press, curtain coater, or spraycoater. The coating amount of a clear coating layer is in the range ofpreferably from 0.1 to 1.0 g/m², more preferably from 0.2 to 0.8 g/m²,in terms of solids per one side.

(3) Pigment Coating Liquid

The pigment coating liquid is a composition comprising a white pigmentas a main component. Examples of a white pigment include commonly usedpigments such as calcium carbonate, kaolin, clay, fired kaolin,amorphous silica, zinc oxide, aluminum oxide, satin white, aluminumsilicate, magnesium silicate, magnesium carbonate, titanium oxide, andplastic pigments.

The content of a CM-modified MFC in a pigment coating liquid ispreferably in the range of 1×10⁻³ to 1 part by weight based on 100 partsby weight of a white pigment. When this content falls within theaforementioned range, there can be obtained a pigment coating liquidhaving moderate water retention ability without showing a significantincrease in viscosity.

The pigment coating liquid contains a binder. Examples of a binderinclude, but are not limited to: different types of starches, such asoxidized starch, cationic starch, urea-phosphoric acid esterifiedstarch, etherified starch (e.g., hydroxyethyl etherified starch), anddextrin; different types of proteins, such as casein, soybean protein,and synthetic protein; polyvinyl alcohol; cellulose derivatives such ascarboxymethylcellulose and methylcellulose; conjugated diene polymerlatexes, such as styrene-butadiene copolymer and methylmethacrylate-butadiene copolymer; acrylic polymer latexes; and vinylpolymer latexes such as ethylene-vinyl acetate copolymer. Such bindersmay be used alone, or two or more thereof may be used in combination. Itis preferable to use a starch-based binder and a styrene-butadienecopolymer in combination.

The pigment coating liquid may contain different auxiliary agentscommonly used in the field of paper production, such as dispersant,thickener, antifoamer, colorant, antistatic agent, or antiseptic agent.

By coating one or both sides of a base paper with a pigment coatingliquid by a known method, a pigment coating layer can be formed. Fromthe viewpoint of coating suitability, the solids concentration of apigment coating liquid is preferably in the range of about from 30 to70% by weight. One, two or three or more pigment coating layers may beformed. When there are two or more pigment coating layers, it is onlynecessary that any one of the layers should be formed with a pigmentcoating liquid comprising a CM-modified MFC. The coating amount of apigment coating layer is adjusted as appropriate depending on theintended use, but in the case of production of a coated paper forprinting, said coating amount is not less than 5 g/m², preferably notless than 10 g/m², per one side in total. The upper limit of thiscoating amount is preferably not more than 30 g/m², more preferably notmore than 25 g/m².

(3) Dry Solid

The composition of the present invention can be dried into a dry solid.In particular, a dry solid (base paper, clear coating layer, pigmentcoating layer) obtained by drying a water dispersion comprising a rawmaterial pulp, a water-soluble polymer, a white pigment or theCM-modified MFC of this invention has both strength and pliableness. Thereason for this is not known but is presumed to be as follows. Since awater dispersion of the CM-modified MFC of this invention is defibratedin a gentler manner than a CNF which is defibrated to thesingle-nanometer level, the CM-modified MFC of this invention isdispersed in water while it has a fibrillated surface but retains itsfiber structure. Therefore, a dry solid obtained by drying such a waterdispersion contains a fiber network which is reinforced by hydrogenbonds formed between fibrillated fibers, and thus combines strength andpliableness. Said dry solid can be used as a composition when water isadded thereto.

3. Paper Comprising a CM-Modified MFC

Since a pulp slurry comprising the CM-modified MFC of the presentinvention has high water retention ability, a paper made from such apulp slurry has high paper strength and high air resistance. Also, apaper having a pigment coating layer or clear coating layer formed froma pigment coating liquid or clear coating liquid comprising theCM-modified MFC of this invention shows a reduced degree of penetrationof a binder into a base paper, and thus has high coating layer strengthand high air resistance.

A paper comprising the CM-modified MFC of the present inventionpreferably has a base weight of from 10 to 400 g/m², more preferablyfrom 15 to 100 g/m². A base paper used to produce a paper comprising theCM-modified MFC of this invention may be composed of a single layer orof multiple layers. A paper made from a pulp slurry comprising theCM-modified MFC has a base paper layer comprising the CM-modified MFC.When the paper has multiple base paper layers, it is only necessary thatat least any one of these layers should comprise the CM-modified MFC.Further, the ash content of said paper varies with the presence orabsence of a pigment coating layer, but this ash content is preferablyin the range of from 0 to 30% for a paper having no pigment coatinglayer (i.e., base paper or clear coated paper), and in the range of from10 to 50% for a paper having a pigment coating layer.

A paper comprising the CM-modified MFC may have a clear coating layerdepending on the need. Also, a paper comprising the CM-modified MFC maybe subjected to surface treatment or other treatments by a known method.

EXAMPLES

Hereunder, the present invention will be described by way of examples.Analysis of physical properties was performed according to the followingprocedures.

Average fiber length, average fiber diameter: A 0.2% by weight slurrywas prepared by adding ion exchange water to a sample and determined forthese properties using a fractionator produced by Valmet K.K.

Canada standard freeness (c.s.f.): This property was determinedaccording to JIS P 8121-2:2012.

Electrical conductivity: A water dispersion with a sample (e.g.,CM-modified MFC) concentration of 1.0% by weight was prepared anddetermined for electrical conductivity at pH 8 using a portableelectrical conductivity meter produced by Horiba Ltd.

Base weight: This property was determined according to JIS P 8223:2006.

Bulk thickness and bulk density: These properties were determinedaccording to JIS P 8223:2006.

Specific burst index: This property was determined according to JIS P8131:2009.

Specific tensile strength: This property was determined according to JISP 8223:2006.

Tensile elongation at break and specific tensile energy absorption:These properties were determined according to JIS P 8223:2006 and JIS P8113:1998.

Short-span specific tensile strength: This property was determinedaccording to JIS P 8156:2012.

Air resistance: This properties was determined according to JIS P8117:2009 using an Oken air resistance-smoothness tester.

[Examples A1, A2] Preparation of CM-Modified MFCs

A stirrer capable of mixing pulp was charged with 200 g by dry weight ofa pulp (NBKP (softwood bleached kraft pulp), produced by Nippon PaperIndustries Co., Ltd.) and 111 g by dry weight of sodium hydroxide, andwater was added to give a pulp solids content of 20% by weight.Thereafter, the mixture was stirred at 30° C. for 30 minutes, and then216 g of sodium monochloroacetate (in terms of active component content)was added thereto. The resulting mixture was stirred for 30 minutes,heated to 70° C., and further stirred for 1 hour. Thereafter, thereaction product was taken out, neutralized and washed to obtain aCM-modified pulp of Example A1 having a degree of carboxymethylsubstitution per glucose unit of 0.25.

The obtained CM-modified pulp was dispersed in water to form a 4% byweight water dispersion, which was treated in a high-speed disintegrator(product name: TopFiner, produced by Aikawa Iron Works Co., Ltd.) toobtain a CM-modified MFC of Example A2. The physical properties of theobtained CM-modified MFC are shown in Table 1.

Comparative Examples A1, A2

A NBKP pulp treated by the same procedure as in Example A2 was obtained,except that a non-CM-modified pulp (NBKP, produced by Nippon PaperIndustries Co., Ltd.) was used and the high-speed disintegrator wasreplaced with a single-disc refiner (product name: 14 Inch Labo Refiner,produced by Aikawa Iron Works Co., Ltd.). The physical properties of thetreated pulp and the NBKP used as a raw material are shown in Table 1.In this table, the NBKP used as a raw material is denoted as ComparativeExample A1.

Example B1

96% by weight of a corrugated waste paper (produced by Nippon PaperIndustries Co., Ltd.) and 4% by weight of the CM-modified MFC (<10 mLc.s.f.) prepared in Example A2 were mixed to give a mixed pulp with asolids concentration of 0.8% by weight. Based on the total amount of themixed pulp, 1.0% by weight of aluminum sulfate, 0.15% by weight ofpolyacrylamide, and 0.2% by weight of a sizing agent were added toprepare a stock. The prepared pulp slurry was used to make a handmadesheet with an aim to achieve a base weight of 100 g/m², and the handmadesheet was subjected to analysis. The handmade sheet was made accordingto JIS P 8222.

Comparative Examples B 1, B2

Handmade sheets were made and analyzed by the same procedure as inExample B 1, except that no CM-modified MFC was used. The corrugatedwaste paper used in Comparative Example B1 was of the same lot as thatused in Example B 1, and the corrugated waste paper used in ComparativeExample B2 was of the same lot as that used in Comparative Example B3.

Comparative Example B3

A handmade sheet was made and analyzed by the same procedure as inExample B 1, except that the pulp obtained in Comparative Example A2 wasused instead of a CM-modified MFC. The physical properties of thishandmade sheet are shown in Table 2.

TABLE 1 Ex. A1 Ex. A2 Com Ex. A1 Com Ex. A2 Type CM-modified MFCUntreated NBKP Treated NBKP Raw material CM-modified pulp (Na) NBKPAverage fiber length mm 0.77 0.58 1.72 1.62 Average fiber diameter μm15.2 14.2 16.6 17.6 CSF ml 52 ≤10 620 156 Electrical conductivity mS/m28 53 5 13

TABLE 2 Com. Ex. B1 Ex. B1 Com. Ex. B2 Com. Ex. B3 Raw material TypeCorrugated waste paper Amount added wt. % 100 96 100 96 Refiner-treatedpulp — Ex. A2 — Com. Ex. A2 Amount added wt. % 0 4 0 4 Bulk thickness mm0.161 0.151 0.163 0.159 Bulk density g/cm³ 0.63 0.67 0.62 0.64 Specificburst index kPa · m²/g 3.27 3.82 3.11 3.13 Specific tensile strength N ·m/g 39.6 51.3 39.6 41.7 Tensile elongation at break % 2.2 2.6 2.1 2.1Specific tensile energy absorption J/kg 629 956 577 612 Short-spanspecific tensile strength kN · m/kg 23.9 27.6 23.7 24.8 Air resistance(Oken) sec 25 82 25 31

It is apparent that the paper of the present invention has excellentpaper strength and air resistance.

The invention claimed is:
 1. A carboxymethylated microfibrillatedcellulose fiber having a Canada standard freeness of less than 200 mL anaverage fiber diameter of not less than 500 nm, and an electricalconductivity between 10 and 100 mS/m, as measured at pH 8 in the form ofa 1% by weight solids concentration water dispersion.
 2. Thecarboxymethylated microfibrillated cellulose fiber according to claim 1,wherein the electrical conductivity is between 10 and 70 mS/m, asmeasured at pH 8 in the form of a 1% by weight solids concentrationwater dispersion.
 3. The carboxymethylated microfibrillated cellulosefiber according to claim 2, having a degree of substitution of from 0.01to 0.50.
 4. The carboxymethylated microfibrillated cellulose fiberaccording to claim 3, having a cellulose type-I crystallinity of notless than 50%.
 5. The carboxymethylated microfibrillated cellulose fiberaccording to claim 1, having a degree of substitution of from 0.01 to0.50.
 6. The carboxymethylated microfibrillated cellulose fiberaccording to claim 1, having a cellulose type-I crystallinity of notless than 50%.